1) Field of the Invention
The present invention relates to an exhaust gas cleaning device constructed of a metal-made honeycomb core body for carrying an exhaust gas cleaning catalyst. As a cleaning means for exhaust gas from a motor vehicle, the exhaust gas cleaning device is generally installed at an intermediate point of an exhaust pipe.
More specifically, this invention is concerned with an exhaust gas cleaning device of the above sort, which is employed under severe conditions. The exhaust gas cleaning device features the use of a metal-made honeycomb core body with improved resistance to deformations and breakage by thermal expansion and stresses.
2) Description of the Related Art
Exhaust gas cloaning devices of the above soil have conventionally been fabricated in the following manner. Namely, a planar band made of a heat-resistant steel sheet and a corrugated band obtained by forming a similar steel sheet into a wavy or sinuous form are superposed one over the other in a contiguous relationship. They are rolled into a multi-layered spiral form, thereby forming a honeycomb-shaped multi-layered composite body (hereinafter called the "honeycomb core body") axially defining a number of network-patterned gas flow passages for allowing exhaust gas to flow therethrough. The honeycomb core body is then inserted into a cylindrical metal casing which is open at in both ends thereof. The honeycomb core body and metal casing are thereafter firmly joined together by brazing or soldering, so that they can withstand thermal stresses and deformations due to the high temperature of exhaust as, exothermic reactions between exhaust gas and the catalyst and the like and further vibrations while the associated motor vehicle is running. Needless to say, the planar and corrugated bands which make up the honeycomb core body are firmly joined together at contacts therebetween by any one of various suitable methods, for example, by welding or brazing.
There is however, a recent move toward constructing an exhaust gas cleaning device without a separately-fabricated metal casing, in other words, with a metal-made honeycomb core body alone from the stand point of the price competition with conventional cordierite ceramic carriers. The omission of such a metal casing naturally leads to a reduction in the cost for inserting a metal-made honeycomb core body in the metal casing and then uniting them together, namely, in the so-called canning cost, whereby a substantial cost merit can be brought about
However, when the metal casing is omitted, the outermost peripheral wall of the honeycomb core body is exposed directly to the outside air so that internal heat is radiated from the outermost periperal wall of the honeycomb core body. A considerable temperature difference therefore occurs between the outermost peripheral wall of the honeycomb core body and an adjacent core portion which is located a little inside the outermost peripheral wall. In an exhaust gas cleaning device making use of a honeycomb core body of the above sort, the flow velocity of exhaust gas is faster through an axially central portion. Acordingly, the temperature of the axially central portion becomes higher than that of a peripheral core portion due to reactions of exhaust gas, which reactions are induced by a catalyst carried on the honeycomb core body. As a result, deformation forces and strains which are caused by thermal expansion and thermal stresses which occur in and/or between members (i.e., planar band and corrugated band) making up the honeycomb core body are concentrated on the outermost peripheral wall of the honeycomb core body. This may lead to cracking and breakage of the outermost peripheral wall of the honeycomb core body and therefore is very dangerous. Moreover, this may result in cracking and breakage of the planar band and corrugated band in the core portion located adjacent the outer peripheral wall of the honeycomb core body and may also accelerate the separation of a catalyst layer carried on the honeycomb core body. As a result, the cleaning performance of exhaust gas may be lowered.